Various surfaces, particularly metallic surfaces, tend to corrode in a moist environment and thus require protection therefrom.
Commonly used methods of passivating surfaces against corrosive attacks are oxidative treatments for forming oxide layers on surfaces, and phosphatizing or chromatizing treatments, in which case so-called conversion coatings are produced, which in the latter case consist of toxic chromates.
In the case of particles such as occur, for example, in metallic pigments in the form of aluminum flakes, it is difficult to produce an adequately dense coating using conventional processes, particularly because the coefficient of thermal expansion of the metal, in this case aluminum, differs from the coefficients of expansion of aluminum oxide so that the protective coating tends to form cracks when subjected to thermal stress.